Depending on the type of printing ink and the underlying special drying process, different types of printing press installations are known, in particular planographic presses, such as lithographic presses, rotary presses, offset presses, flexographic presses, and the like, which process sheet- or web-shaped printing substrates, in particular paper, cardboard, carton, and the like, which initiate or promote an adhesion of the ink to the printing substrate, in that radiant energy, in particular in the form of light, is fed to the printing ink located on the printing substrate.
The so-called UV inks cure by polymerization, which is triggered by photoinitiation using ultraviolet light. On the other hand, solvent-containing printing inks, which are able to undergo both a physical as well as a chemical drying process, are very common. Physical drying encompasses the evaporation of solvents and the diffusion into the printing substrate (absorption), while chemical drying or oxidative drying is based on the polymerization of the oils, resins, binding agents, or the like, contained in the ink formulations, possibly with the co-action of atmospheric oxygen. The drying processes are generally dependent on one another, since the absorption of the solvents effects a separation between solvents and resins within the binding agent system, so that the resin molecules come closer together and possibly polymerize more easily.
The European Patent 0 355 473 A2, related to U.S. Pat. No. 4,991,506, both of which are incorporated by reference herein, for example, describes a device for drying printed products, which includes a radiant energy source in the form of a laser. The radiant energy is transmitted to the surface of the printing substrates, which are conveyed along a path through the printing press by a transport device, at a position between individual print units or following the last print unit, before or in the delivery unit. In this context, the radiation source can be a laser in the ultraviolet region for UV inks or a laser light source for heating solvent-containing printing inks. The radiant energy source is configured outside of the printing press to prevent parts of the press from being undesirably heated because of dissipation heat that cannot be avoided or that cannot be shielded. Here, the disadvantage is, however, that an additional system component must be separately provided for the printing press.
To remove solvents from a solvent-containing printing ink and/or water, it is also known from U.S. Pat. No. 6,026,748, for example, for a printing press to be provided with a drying device having infrared lamps, which emit short-wavelength infrared light (near infrared) or medium-wavelength infrared light. The emission spectrum of lamp light sources is broadband and, therefore, offers a multiplicity of wavelengths. The drawback of such drying devices in the infrared region is that a considerable proportion of the energy absorption takes place in the paper, the ink only being indirectly heated. A rapid drying is only possible by inputting enough energy. In the process, however, there is the danger, inter alia, of the printing substrate drying out unevenly and becoming warped.
In electrophotographic printing technology, it is known, for example, from the German Patent Application No. 44 35 077 A1, hereby incorporated by reference herein, to fix toner to a recording medium by using radiant energy in the near infrared region emitted by diode lasers. A narrow-band light source is used to heat the toner particles, in order to melt them, to form them into a colored coating, and to anchor them to the surface of the recording medium. Since in this spectral region, a considerable number of common paper grades have broad absorption minima, it is possible that a predominant share of the energy is directly absorbed into the toner particles.
Moreover, it is known from the German Patent Application No. DE 101 07 682 A1, hereby incorporated by reference herein, that an electrophotographic printing press or copy machine can have a plurality of fixing devices for toner, each of the fixing devices having a wavelength range of electromagnetic radiation which corresponds to a maximum absorption wavelength of the type of toner assigned to this fixing device, but exhibiting no or only little absorption at absorption wavelengths of other types of toner.
However, the simple knowledge of the window in the paper absorption spectrum cannot be directly exploited in printing technology that uses solvent-containing printing inks, since, as described above, there are other underlying chemical and/or physical drying processes. In the context of the present invention, the concept of solvent-containing printing ink connotes, in particular, inks whose solvent constituents may be of an aqueous or organic nature, which are derived from binding agent systems, which are able to be oxidatively, ionically or radically polymerized. An energy input for drying solvent-containing printing inks is intended to assist or promote the effect of evaporation of the solvent and/or the effect of absorption into the printing substrate and/or the effect of polymerization, unwanted secondary effects, such as a too intense heating of the solvent-containing printing ink, which can lead to a breakdown of components, or overheating of the solvent, being avoided at the same time. It is not intended for the energy input to be introduced just for melting particles, as in the case of fixing the toner.
German Patent Application No. DE 102 34 076 A1, hereby incorporated by reference herein, describes admixing an infrared absorber—a substance which absorbs in the near infrared spectral region—to a printing ink to be used for printing in a print unit. A narrow-band radiant energy source, preferably a laser light source, configured downstream from the printing nip, is used to illuminate the printing ink on the printing substrate. Supplying light of one wavelength that is essentially resonant to an absorption wavelength of the infrared absorber, effects, renders possible, or promotes an energy input into the printing ink in a way that dries the printing ink. At the same time, in order to minimize or avoid energy input into the printing substrate, the wavelength of the radiant energy source and the absorption wavelength of the infrared absorber are selected in such a way that the wavelength used is not resonant to water.